1. Field of the Invention
The present invention relates to a printing apparatus and a printing method for employing a print head where a plurality of print elements are arranged and printing an image on a print medium.
2. Description of the Related Art
Generally, a so-called serial scan ink jet printing apparatus includes a carriage on which a print head serving as printing means is mounted, a conveying unit for conveying a print medium, and a controller for controlling these components. For printing an image on the print medium, the printing apparatus repeats a printing operation, for ejecting ink through a plurality of nozzles in the print head while moving the print head in the main scan direction, and an operation for conveying the print medium in the sub-scan direction crossing the main scan direction. Ejection energy generation elements, such as electrothermal transducing elements or piezoelectric elements, are provided for the individual nozzles, and when the ejection energy generation elements are driven, ink is ejected through ejection ports formed at the tips of the nozzles. The nozzles serve as printing elements for applying ink to the print medium.
An example driving method for the print head is a time division driving method (block driving method) for employing time division for a plurality of nozzles for each block. For example, for a print head wherein 128 nozzles of nozzle numbers 1 to 128 are formed into arrays in the main scan direction, which is perpendicular to the sub-scan direction, the 128 nozzles are divided into eight blocks from the first to the eighth, and nozzles of nozzle numbers 1, 9, 17, . . . and 121 are assigned to the first block. Similarly, the nozzles of nozzle numbers 2, 10, 18, . . . and 122 are assigned to the second block, the nozzles of nozzle numbers 3, 11, 19, . . . and 123 are assigned to the third block, and the nozzles of nozzle numbers 4, 12, 20, . . . and 124 are assigned to the fourth block. The same assignment is performed for the fifth to the eighth blocks. Assume that, using this print head, a ruled line having a width equivalent to one dot in the sub-scan direction was printed at a resolution of 1200 dpi in the main scan direction. In this case, due to a drive time difference for the first to the eighth blocks, the landing positions of ink droplets ejected from the nozzles assigned to the individual blocks would deviate in the main scan direction. Thus, when ink is ejected from the nozzles of nozzle number 1 and nozzle number 8, the landing positions of ink droplets deviate, in the main scan direction, a distance of 21 μm which is equivalent to about 1/1200 dpi.
This deviation in the landing positions is seldom identified as an image defect in a case wherein only a single print head is employed to print a single-color image by a one-pass printing method for scanning a predetermined print area by moving the print head one time. However, in a case wherein a plurality of print heads are employed to print an image by a multi-pass printing method for scanning a predetermined print area by moving the print heads a plurality of times, one raster image is printed using a plurality of different nozzles, and therefore a belt-shaped density unevenness would appear.
Assume that image printing was performed by the multi-pass printing method while employing two print heads, and that because of a printing head mounting error, the landing positions of ink droplets ejected from the nozzles of the print heads were displaced, a distance equivalent to one pixel in a direction in which the nozzles are arrayed (sub-scan direction). In this case, combination blocks, to which the nozzles of the two print heads for forming dots on a single raster belong, are changed. When the nozzles for forming dots on the single raster belong to different blocks, the landing positions of the ink ejected from these nozzles deviate relative to each other, and the overlapping states of dots formed by the ink are varied. When the overlapping states of the dots are varied, the density of a printed image is changed in accordance with a block drive period.
In Japanese Patent Laid-Open No. 2001-071466, a construction for multi-pass printing is described wherein a plurality of nozzles used for printing the identical raster are driven at two or more different block drive timings. Also, in Japanese Patent Laid-Open No. 2001-071466, a method is described for proportionally distributing drive blocks to individual rasters. Specifically, numbers indicating the order for driving are provided for the individual blocks, and, for all of the rasters, the same value is set as the total of the numbers for the blocks to which the nozzles employed for printing a single raster belong. Furthermore, in Japanese Patent Laid-Open No. 2004-276473, a method is described according to which, for an elongated print head (a connecting head) including a plurality of small print heads partly overlapped in the sub-scan direction, the identical block is set for the nozzles in the overlapping portions of the elongated print head.
However, the technique in Japanese Patent Laid-Open No. 2001-071466 is assumed for multi-pass printing and is not compatible with one-pass printing that employs a plurality of print heads, and further there is no description given concerning mounting errors in the print heads. In addition, in Japanese Patent Laid-Open No. 2004-276473, there is no description given concerning mounting errors in the print heads and multi-pass printing.